BMW offers a Diesel Exhaust Fluid Refractometer under P/N 83 30 2 238 146 at a liste price of $115.
The Selective Catalytic Reduction system (SCR) uses DEF (Diesel Exhaust Fluid) as a catalyst to reduce nitrogen oxide (NOX) in the diesel engine’s exhaust. When used in its proper concentration, DEF breaks down into harmless water and nitrogen in the final stage.
In order for DEF to work properly, it must be in a concentration of 32.5% pure urea and 67.5% de-ionized water. Ions from sources like water pipes and soil make the DEF chemical reaction less effective.
The DEF Refractometer is used to test the concentration of DEF if diluted/weak DEF is suspected.
The refractometer is a simple device that uses the bending of light (refraction) to accurately detect the level of urea.